PROJECT SUMMARY/ABSTRACT Abundant evidence documents that children in high-risk family environments?characterized by a lack of father engagement, poor parental mental health, or parental conflict or violence?face adverse consequences for their socio-emotional development. But what are the factors associated with the large number of children who develop positive outcomes despite high-risk family environments? This project aims to advance knowledge on this question by studying two commonly implemented policies that share a goal of improving child wellbeing through father involvement: paternity leave and joint legal custody. The proposed research is motivated by a large literature on father involvement, which points to paternal engagement during infancy and accessibility during adolescence as important pathways through which fathers can influence child wellbeing. The project uses large- scale population-level administrative data coupled with natural experiments from Sweden and Denmark to produce novel estimates of the causal impacts of these policies on child health and development. The paternity leave analysis focuses on outcomes related to cognition and stress reactivity in early to middle childhood, while the joint custody analysis examines components of child mental and socio-emotional health related to paternal emotional accessibility in adolescence: anxiety, depression, and markers of externalizing behaviors. First, the project applies a regression discontinuity research design to study a Swedish ?daddy-month? reform in 1995, which earmarked one month of parental leave to fathers only. The project analyzes how the resulting increase in paternity leave impacts child health outcomes through age 10, examining maternal mental health as a mediator. Second, the project constructs a new data set that links Danish court records to several other administrative databases and exploits a natural experiment in which otherwise similar families are randomly assigned to judges who differ in their propensity to rule in favor of joint custody. The project studies the effects of joint legal custody on adolescent health outcomes, examining differences in impacts across children in families with and without histories of poor parental mental health or domestic violence. In both analyses, health outcomes are measured using outpatient, inpatient, and pharmaceutical claims data. Results from Sweden will provide a model of how paternity leave may facilitate early father engagement and thereby influence child development, which is relevant for the U.S. where paternity leave is becoming more common in private companies and through state and local policies. Results from Denmark will be relevant to many U.S. states considering making joint custody the legal default. U.S.-based population-level studies on these policies are currently impossible because of the absence of relevant large-scale data and exogenous variation to separate causation from correlation. Results from this project will be helpful for identifying policies and mechanisms that can promote short- and long- term psychosocial adjustment and socio-emotional development for children in high-risk family environments, which is a high priority research gap for the NICHD Child Development and Behavior Branch.